A hysteretic controlled switching regulator is typically used to provide a constant and predetermined output voltage to a load. Input to the hysteretic controlled switching regulator may typically be a fluctuating voltage source. As an example, a commonly used application of a hysteretic controlled switching regulator would be in the conversion of rectifier output in a standard AC adaptor (which can have a significant amount of ripple) to stable DC output that can be used in a wide variety of electronic equipment, such as computers, peripherals, and so forth. By adjusting the duty cycle of the switching, the output voltage can be controlled to vary within a certain range. A hysteretic comparator can be used to adjust the duty cycle.
A problem that can arise with hysteretic controlled switching regulators is that when it is driving a light load, the switching frequency may enter the range of human hearing (i.e., less than 20 kilo-hertz). This may then lead inductive components connected to the switching regulator to generate audible noise. The noise generated can be highly undesirable by users of the electronic equipment, and therefore is typically unwanted.
One solution uses a fixed switching frequency and a forced off time (a time when the switching regulator is forced into the off state) to ensure that the switching frequency does not enter the range of human hearing. By fixing the switching frequency of the switching regulator, it would be impossible for the switching frequency to drift off into hearing range.
One disadvantage of the prior art is that the forced off time and the fixed frequency requires a higher minimum supply voltage due to the fact that there is a maximum ratio for on time to off time.
A second disadvantage of the prior art is that it operates at a fixed frequency and cannot adjust its operating frequency dynamically. Therefore, to make adjustments to the operating frequency, designers need to make the adjustments.